This invention relates generally to an electro-optical display device, and more particularly, to a phase transition mode liquid crystal display device.
In a twisted nematic liquid crystal display, an LED display or the like, predetermined voltages e.g., about 3 volts, are applied to selected display picture elements and 0 volts are applied to nonselected display picture elements in order to obtain the desired display information. However, in a kind of electro-optical display device such as a phase transition mode liquid crystal display device, desired voltages other than 0 volts may be applied to the nonselected display picture elements, while predetermined voltages are applied to the selected elements. The phase transition mode liquid crystal display element may take three phases, i.e., a homeotropic state, which is referred to as an H-state hereinafter, a focal-conic state referred to as an F-state hereinafter, and a granjuane state referred to as a G-state hereinafter, according to the applied voltage. The G-state is a transparent state which is caused under a non-electric field condition. The G-state is changed into the F-state, which is a light scattering state, when a voltage is applied thereto. The F-state is changed into the H-state, which is a transparent state, when a stronger voltage is applied thereto. It is possible to display information by using the transparent G-state and the light scattering F-state. In this kind of display system, a static driving method similar to that for driving a usual twisted nematic mode liquid crystal display device may be used. Namely, a voltage of square wave shape having an effective value .+-.V.sub.F, which holds the liquid crystal in the F-state, is applied between selected display segment electrodes and a common electrode and zero volts are applied between nonselected display segment electrodes and the common electrode. However, it takes a relatively long time for the transition from the F-state to the G-state and the display exhibits an inferior transparency in the G-state.
Generally, a digital integrated circuit is used as a liquid crystal driving circuit in order to reduce the power consumption and to increase the space for other elements. In the digital integrated circuit, the voltage level for driving the liquid crystal is obtained by boosting or dropping the source voltage using condensers, so that the level is limited to an integral number of times or integral fraction of the voltage of a power source, e.g., the voltage of a battery.
According to the electric property of the phase transition mode liquid crystal, the voltage needed to exhibit the F-state or the H-state is not equivalent with the voltage level obtained by boosting or dropping the source voltage. The picture quality, e.g., the contrast, of the phase transition mode liquid crystal element depends on an applied effective voltage.